JIGSAW LEARNING: CONTEMPORARY CARBON CYCLE
Part 1: Land
How does life on land affect contemporary atmospheric CO2?
Most people know that plants use solar energy to convert inorganic CO2 into longchain organic molecules, and that this process of photosynthesis is the base of the
food chain on Earth. At first glance then, perhaps “trees are the answer” as the
bumper sticker assures us? But of course life and growth are inevitably followed by
death and decay, so in the natural order whatever carbon is removed from the
atmosphere to make plants is soon replaced by the respiration of herbivores and
bacteria.
For plants and soils and animals on land to have a lasting impact on CO2 requires not
just that plants take up some CO2, but rather that the total amount of carbon stored
on land should change. To permanently remove CO2 from the air, the amount of
carbon in biomass and dead material has to increase permanently, and vice versa.
Life on land can only remove carbon from the atmosphere if living things grow
faster than they decay for a long time.
Yet to the great surprise of many biologists, the total amount of living and dead
carbon on land has in fact been increasing for decades, despite some people’s
efforts to “pave paradise and put up a parking lot.” We know that life on land is a
sink for atmospheric CO2 from the rate of CO2 growth, from the stable isotope ratio
of CO2 gas, and from other data. Ecologists fought with oceanographers and
atmospheric scientists for many years about this, but the ice cores settled it in the
1980s. Believe it or not, stuff is growing faster than it’s dying!
This is even weirder than it seems. If we were to fertilize a patch of forest and make
it grow faster, we would soon have bigger trees but also more dead branches and
leaf litter on the forest floor. More soil carbon for microbes and insects in the soil to
digest. And pretty soon the respiration from all that microbial decomposition would
catch up to the increased photosynthesis. At that point there would be more carbon
stored in the ecosystem, but we’d have reached a new steady state in which the rate
of growth was once more equal to the rate of decay. Yet since at least 1960, life on
land has been a sink of atmospheric CO2 nearly every single year.
How can this be? There are four leading causes:
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CO2 Fertilization due to rising CO2 levels in the air
Fertilization by nitrogen and other nutrients
Abandonment of formerly cleared land (reforestation), and
Encroachment of woody plants into the Arctic due to Boreal warming
CO2 fertilization is easily observed in a test tube or greenhouse. The chemical
reactions that fix CO2 in chloroplasts simply run faster in a high-CO2 environment as
expected from chemical equilibrium concepts. But it’s surprising that this plant-level
process can be sustained over huge areas over many decades. Most plant growth is
limited by other things than CO2: light, water, nutrients, the length of the frost-free
season, pests, disease, etc. Well-watered and pampered greenhouse plants might
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JIGSAW LEARNING: CONTEMPORARY CARBON CYCLE
Part 1: Land
respond to elevated CO2 when they are fat and happy, but one expects whole
ecosystems to grow at slower rates limited by environmental factors. Outdoor
experiments have shown that doubling ambient CO2 in forests usually produces a
temporary burst of growth, but that after a few years the treated plots revert to
slower growth as nutrient or other limitations kick in.
As it turns out, people have been helping out with nutrient limitations as well! Plant
growth in most natural ecosystems is limited by nitrogen (meaning if we add
nitrogen, they grow faster). Since the invention of chemical fertilizers in 1908, we’ve
been converting inert N2 from the air into bio-available nitrogen and pouring it all
over the biosphere. The Green Revolution has allowed us to feed ever-growing
numbers of people, but not all the fertilizer has remained in crop fields. Some
inevitably blows on the wind and runs off in streams. Besides intentional fertilizer,
we fix nitrogen in industrial and automobile combustion: burning air by
combining N2 + O2 to make NO and NO2 that oxidize in clouds to NO3. Plumes of
dilute Miracle Gro drift downwind of the Ohio Valley, Western Europe, and Eastern
Asia. Much of this fixed nitrogen falls onto forests that are already CO2-fertlized, and
they sequester more and more carbon in wood, leaf, and root.
As a teenager in Massachusetts I walked in lush green forests with closed canopies
of maple, birch, and beech that hid the sky. Yet deep in those woods were miles and
miles of carefully constructed stone walls. Why would somebody go to the trouble of
building all those lovely walls? Because that land wasn’t a forest in the 19th Century!
Nearly all of New England was farms and pastures for centuries, but as agriculture
was displaced by industry and then offices, farms were abandoned and the forest
regrew. Similar stories of agricultural abandonment and forest regrowth are told
across much of the developed world. Every molecule of wood in Vermont was made
from CO2 molecules.
The Arctic has warmed more than twice as fast as the rest of the world over recent
decades as polar ice melts and allows more solar radiation to be absorbed. In many
places in Alaska, Canada, northernmost Europe, and Siberia there are 50% more
frost-free days each summer than there were 50 years ago. This has allowed woody
shrubs and trees to invade into land that could previously support only
herbaceous tundra. The woody plants store a lot of organic carbon that used to be
CO2.
All of the big land sinks come with expiration dates. Nitrogen fertilizer only
makes plants grow faster until the demand is met. Further additions will only make
NO3-rich streams. Once the forests of New England have regrown to maturity, the
rate of death will catch up as it does for all of us, and those forests will stop
sequestering additional CO2. Making the Arctic a bit warmer can push the treeline
north and store carbon in new Boreal Forests, but a lot more warming will melt the
permafrost and expose thousands of years’ worth of stored carbon to microbial
decay. Even CO2 fertilization will eventually run its course, though we should
probably hope that fraction of the land sink can continue subsidizing our fossil fuel
emissions for a few decades to come.
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